The improvement of existing therapeutics and the development of novel therapeutics to treat and/or prolong survival of cancer patients has been the subject of continuing research in the scientific community. Although certain of these efforts have been directed to more “traditional” chemotherapeutics (e.g., Paclitaxel and other small molecule and/or natural product based therapies) that act by killing malignant cancer cells, it has also been a long-standing goal (Lanzavechis, Science, 260, 937-944; Pardoll et al., Curr. Opin. Immunol. 1993, 5, 719-725; Livingston et al., Curr. Opin. Immunol. 1992, 4, 2; Dranoff et al., Proc. Natl. Acad. Sci, USA 1993, 90, 3539; M. H. Taoet et al., Nature, 1993, 362, 755; T. Boon, Int. J. Cancer 1993, 54, 177) to develop an anti-cancer vaccine to induce an anticancer response. Although cancer vaccines have thus far been perceived as a mode of treatment subsequent to the detection of the disease (for example, by providing an enhanced immunological response), it would be most desirable to develop a selective synthetic vaccine that would be able to provide enhanced protection against tumor recurrence and metastasis when the tumor burden has been rendered minimal through surgery, radiation or other chemotherapeutic treatment.
In general, tumor immunotherapy is based on the theory that tumors possess specific antigens that can be recognized when presented to or processed by a properly trained immune system. The goal for the development of an effective anticancer vaccine is to break the tolerance which the immune system has for these antigens expressed mainly or exclusively by the tumor, by presenting glycoconjugates as versions of immunostimulatory antigens, to induce an effective immune response. In an effort to achieve this goal, identified cancer carbohydrate antigens such as TF, Tn, sTN, KH-1, Ley and Globo-H have been carefully characterized as being over-expressed at the surface of malignant cells in a variety of cancers (breast, colon, prostate, ovarian, liver, small cell lung and adenocarcinomas). In addition, they have been immunocharacterized by monoclonal antibodies and therefore have relevant serological markers available for immunological studies. Such studies have suggested that patients immunized in an adjuvant setting with carbohydrate-based vaccines produce antibodies reactive with human cancer cells, and that the production of such antibodies prohibits tumor recurrence and correlates with a more favorable diagnosis (see, Livingston et al., J. Cancer Res. 1989, 49, 7045; Ragupathi, G. Cancer Immunol. Immunother. 1996, 43, 152). Additionally, the isolation and careful structural identification of specific carbohydrate antigens overexpressed in cancer cells has provided a framework for an attack using carbohydrate-based tumor immunotherapy (For reviews see (a) Hakomori, S.; Zhang, Y. Chem. Biol. 1997, 4, 97; (b) Toyokuni, T.; Singhal, A. K. Chem. Soc. Rev. 1995, 24, 23 and references therein).
A major drawback in using carbohydrate epitopes, however, is that they are generally not readily available by isolation from natural sources. For example, the immense difficulties associated with their purification from natural sources render them virtually nonavailable as homogeneous starting materials for a clinical program. Thus, the incorporation of these naturally occurring epitopes into carrier proteins or any favorable molecular context via conjugation for eliciting a therapeutically useful immunological response is inefficient at best, and often virtually impossible. Therefore, to effectively study these vaccines as therapeutic agents, sufficient material can only be obtained by total chemical synthesis.
In an effort to remedy this problem, one of the continuing research efforts is the development of anti-cancer vaccines that incorporate fully synthetic carbohydrate moieties (For a review, see Danishefsky, S. J.; Allen, J. R. Angew Chem. Int. Ed. 2000, 39, 836-863). One strategy for the development of synthetic anti-cancer vaccines involves the total synthesis of the carbohydrate epitope and its subsequent covalent bioconjugation to carrier protein. The vaccine constructs are then subjected to appropriate mouse immunization studies, with the ultimate goal of advancing to human clinical trials. This strategy has resulted in several fully synthetic tumor associated carbohydrate-based vaccines which are at various stages of advanced pre-clinical and clinical processing. In fact, a Globo-H vaccine is undergoing clinical evaluation for the treatment of prostate and breast carcinomas at the phase II level (see, for example, Ragupathi et al., Angew. Chem. Int. Ed Engl. 1997, 36, 125) while a Lewisy antigen-based vaccine, already tested in ovarian cancer, is awaiting more extensive follow-up evaluation (see, Kudryashov et al. Cancer Immunol. Immunother. 1998, 45, 281).
Although several synthetic constructs have been developed in recent years, as described above, and in other references described herein, there remains a need for the further investigation to develop novel constructs capable of eliciting a more sustained or effective (and preferably selective) immune response. Clearly, in an effort to achieve this goal, it would be useful to develop improved and/or novel synthetic methods to access heretofore synthetically unavailable antigenic components (e.g., more complex antigenic components such as fucosyl GM1, clustered epitopes and similar structures), or to access non-natural structures derived from naturally occurring structures for further immunologic and therapeutic studies.